python: add S2PointVector type.
python: add S2PointVector type.

yjwong/s2-geometry-library,laramiel/s2-geometry-library,yjwong/s2-geometry-library,micolous/s2-geometry-library,Noctem/pogeo,Noctem/pogeo,micolous/s2-geometry-library,yjwong/s2-geometry-library,yjwong/s2-geometry-library,laramiel/s2-geometry-library,micolous/s2-geometry-library,micolous/s2-geometry-library,laramiel/s2-geometry-library,Noctem/pogeo,Noctem/pogeo